1. Field of the Invention
This invention relates to electrical switching apparatus having arcing contacts which open after the main contacts to protect the main contacts from damage and wear caused by arcing. More particularly, it relates to the construction of the arc runner which transfers the arc from the arcing contacts to an arc chute where it is extinguished.
2. Background Information
Electrical switching apparatus for power distribution systems includes devices such as, for instance, circuit breakers, network protectors, transfer switches and disconnect switches. Power circuit breakers are typically used to connect a power distribution network to a power source. Such power circuit breakers must be able to withstand high currents for a period of time without tripping to give circuit breakers in the network time to respond and isolate the fault thereby localizing disruption of service. Thus, by the time the power circuit breaker responds, it may have to interrupt a sizable current. This results in the drawing of an arc as the circuit breaker contacts open. It is known to provide an arc chute adjacent to the opening path of the circuit breaker contacts. The arc chute is constructed of a number of spaced plates extending transverse to the arc. As the contacts open, the arc is transferred by electromagnetic forces to the arc plates which cool the arc and increase the arc voltage by breaking it up into sections, both of which help to extinguish the arc.
In a power circuit breaker with "heel-toe" contact parting action, the arcing end of a copper contact finger ("toe") comes in contact with a stationary copper arcing contact ("toe block") after the breaker has begun to open. The main contacts then part followed by parting of the arcing contacts resulting in the striking of an arc between the copper arcing contacts. This protects the main contacts from damage due to arcing. Typically, an arc runner is mounted on top of the stationary arcing contact in order to provide a surface for the arc to run toward the arc chute. The arc is formed on the arcing contact and must travel across the joint to the arc runner. At low currents, the electromagnetic force on the arc may not be adequate to force the arc to cross this joint. One end of the arc may remain on the stationary arcing contact, severely eroding the contact. If the arc does not move onto the arc runner it will not reach the arc chute in time for the breaker to interrupt.
Also, when the arc is created on the arcing contact, it is more likely to travel along a sharp edge or corner of the part. Arc runners often have a slot up the center of the part to provide an attractive edge for the arc to run along. The edge of the slot encourages the arc to travel up the center of the arc runner, engaging the arc chute near the center and extinguishing the arc sooner. At lower current levels, the arc may be attracted to the laterally extending edge of the stationary arcing contact instead of the slot in the arc runner. This may prevent the arc from running up the arc runner or cause the arc to run to one side of the pole where it may track along the inside wall of the arc chamber.
There is a need therefore for electrical switching apparatus with an improved arrangement for extinguishing arcs generated during current interruption.
There is a more specific need for such an improved arrangement for directing the arc from the stationary arcing contact into an arc chute.